Using a large aperture like f-2.8 can render the foreground and/or the background out of focus.
Most new users of SLR cameras learn that those f-stops on the camera control the size of the hole that lets light that pass through the camera’s lens and onto the sensor; the bigger the number, the smaller the opening; the smaller the number the larger the opening. In low light situations you may need a large opening like f2.8 to allow sufficient light to create a properly exposed photo. On bright days, a small opening like f22 limits the amount of light entering the camera, preventing an overexposed shot.
But this article isn’t about when to use which f-stops; it’s about the other consequences of using different apertures. It’s about what happens to the light when it passes through the camera.
When you shoot wide open, using the largest aperture such as f2.8, the lens has to bend the light so that when it strikes the sensor or film, everything seen through the viewfinder is captured. The focus on the lens determines how the light strikes the sensor. An autofocus camera will adjust the lens so the area covered by the camera’s focus points are in focus. That’s simple enough, except that when shooting wide open, the depth of field (DOF) isn’t very deep. That means that if you are shooting a group photo with a large aperture, the people in the front row will be in focus but those in the back row may be out of focus. Even portraits shot wide open may have the eyes in focus but a nose that isn’t sharp. When shooting landscapes at f2.8, the close foreground may be out of focus.
When shooting using small apertures like f22, the light passes through the lens and the camera doesn’t have to bend it to completely fill the sensor or film. Generally, everything will be in focus when using small apertures.
So, why not shoot using f22 whenever light conditions permit? If you want a snapshot of your subject, using f22 will work fine. But what if you don’t want the background to be in focus. An outdoor portrait with the background in focus detracts from the subject. Often photographers want the background and/or the foreground to be blurry. This blurriness is called “bokeh”. Bokeh isolates the subject and draws the eye to the part of the photo that is sharp. It can even hide distractions in the background since they become so blurred that they are not recognizable.
Don’t let your camera choose the aperture for you. You decide whether you want everything in the photo to be tack sharp or parts of it out of focus. Think about the photo you want to create and use the aperture that will create the effect you want.